MySQL - Counting rows and left join problem

Question

I have 2 tables, campaigns and campaign_codes:

campaigns: id, partner_id, status

campaign_codes: id, code, status

I want to get a count of all campaign_c

Answer 1

I'd opt for something like:

SELECT 
    c.id AS campaign_id, 
    COUNT(cc.id) AS code_count
FROM 
    campaigns c
LEFT JOIN campaign_codes cc on cc.campaign_id = c.id
AND cc.status = 0 -- Having this clause in the WHERE, effectively makes this an INNER JOIN
WHERE c.partner_id = 4
GROUP BY c.id

Moving the AND to the join clause makes the join succeed or fail, crucially keeping resulting rows in where there is no matching row in the 'right' table.

If it were in the WHERE, the comparisons to NULL (where there is no campaign_code) would fail, and be eliminated from the results.